GIMP

http://gnu.org/software/gimpThe GNU Image Manipulation Program is powerful image editing program, and commonly used for photo retouching, image composition, and general graphic design work.

It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc. GIMP also comes with extensive documentation for both users and programmers, including a manual, tutorials, examples of various features, links to other GIMP-related sites, and an extensive list of plug-ins.

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